In recent years, nitride semiconductor light emitting devices such as blue light emitting diodes (LED's) and laser diodes (LD's) using a nitride semiconductor have been put into practice. An LED for emitting blue light using a nitride semiconductor in this manner is formed by, for example, as shown in FIG. 10, laminating a low temperature buffer layer 52 made of GaN or the like in accordance with an MOCVD method, an n-type layer 53 made of GaN or the like, an active layer (light emitting layer) 54 made of a material which determines the wavelength of emitted light, and the band gap energy of which is smaller than that of n-type layer 53, for example, an InGaN based (meaning that the ratio of In to Ga may vary; same hereinafter) compound semiconductor, and a p-type layer 55 made of GaN or the like on a sapphire substrate 51, so that a semiconductor lamination portion 56 is formed, providing a p-side electrode 58 on the surface of semiconductor lamination portion 56 with a light transmitting conductive layer 57 in between, and providing an n-side electrode 59 on the surface of the n-type layer 53, which is exposed by partially etching a part of the semiconductor lamination portion 56. Here, a semiconductor layer having higher band gap energy, such as an AlGaN based (meaning that the ratio of Al to Ga may vary; same hereinafter) compound, may be used in n-type layer 53 and p-type layer 55 on the active layer side, in order to increase the effects of confining carriers.
When such a nitride semiconductor light emitting device is manufactured, a large number of semiconductor light emitting devices are formed simultaneously by dividing a wafer where semiconductor layers have been grown on a substrate into separate chips. As the substrate for this wafer, a sapphire substrate is used usually, as in the example shown in FIG. 10, and an SiC substrate may be used, and thus, a material of which the lattice constant does not match with that of nitride semiconductors is used in most cases. Though in addition to these, there exist examples where an Si substrate is used, Si makes the crystallinity of the grown nitride semiconductor layers deteriorate, and Si absorbs emitted light, and thus the brightness cannot be increased, and therefore, Si is not appropriate for highly bright products having high added value.
When a wafer with this substrate is divided into separate chips, a diamond scribing method, a blade dicing method, dry etching or wet etching are generally used. This diamond scribing method is a method for scratching the rear surface of a wafer and then cracking the wafer, but since sapphire is very hard and cannot be deeply scratched, and in addition, nitride semiconductors are, likewise, very hard, the wafer cannot be cracked with high precision along the intended dividing lines and the cracking extends in unintended directions, and therefore, the ratio of defects increases. In addition, in the case where a wafer is divided into separate chips through blade dicing or etching, since sapphire and nitride semiconductors are very hard, dicing and etching takes a very long time, and at the same time, no mirror surface such as in a plane of cleavage can be obtained, and therefore, a problem arises, particularly in the case where a semiconductor laser is manufactured, such that the reflectance on the resonant surface becomes low.
Meanwhile, in the case where a wafer is divided into chips in this manner, as shown in, for example Japanese Unexamined Patent Publication 2003-338468, a method for forming a denatured region through multiple photons absorption along lines which the substrate is to be divided, by irradiating the wafer with a laser beam of which the focal point is adjusted inside the substrate, forming the cutting starting region along the lines which the substrate is to be divided on the basis of this denatured region and dividing the wafer.
As described above, the light emitting device using the nitride semiconductor is formed by growing nitride semiconductor layers mainly on sapphire substrates. However, sapphire and nitride semiconductor layers are very hard and breakable and are almost impossible to cleave, and therefore, chips are obtained through dicing, dry etching or the like, which takes a very long time, and even when a method for dicing on the way and cracking the substrate is attempted, a problem arises, such that cracks run in unexpected directions, making the yield very low. In particular, though it is necessary in a laser device using the nitride semiconductor to form an end face of a resonant as a surface perpendicular to the direction of resonating of the laser element, if the semiconductor layers cannot be cleaved, no mirror surface can be formed, and an end face of a resonator cannot be formed of a plane of cleavage, and therefore, a problem arises, such that a semiconductor laser having high reflectance and high output cannot be obtained.
In addition, in the method wherein the cutting starting point can be created on the center portion side of the substrate, as described above, by forming a denatured region inside the substrate by irradiating the substrate with a laser beam, it can be expected that the substrate can be cracked relatively uniformly. However, as a result of the diligent research conducted by the present inventor, it was found that clear cleavage cannot be obtained without causing damage to the nitride semiconductor layers, even when GaAs or sapphire is used for the substrate as in the above described Japanese Unexamined Patent Publication 2003-338468.